Alloy steel powder for powder metallurgy and method of producing iron-based sintered body

ABSTRACT

Provided is an alloy steel powder for powder metallurgy that is capable of achieving both high strength and high toughness in a sintered body using the same. An alloy steel powder for powder metallurgy of this disclosure comprises a composite alloy steel powder and graphite powder. The composite alloy steel powder has a specific surface area of 0.100 m 2 /g or more and Mo content in a range of 0.2 mass % to 1.5 mass %, and the graphite powder content with respect to 100 mass % of the alloy steel powder for powder metallurgy is in a range of 0.1 mass % to 1.0 mass %.

TECHNICAL FIELD

This disclosure relates to an alloy steel powder for powder metallurgy preferably used in powder metallurgical techniques, and particularly, it aims at improving strength and toughness of a sintered material using such alloy steel powder for powder metallurgy.

Further, this disclosure relates to a method of producing an iron-based sintered body having excellent strength and toughness produced using the above alloy steel powder for powder metallurgy.

BACKGROUND

Powder metallurgical techniques enable producing parts with complicated shapes in shapes extremely close to product shapes (so-called near net shapes) with high dimensional accuracy, and therefore machining costs can be significantly reduced. For this reason, powder metallurgical products are used as various mechanical structures and parts thereof in many fields.

Further, in recent years, to achieve miniaturization and reduced weight of parts, an increase in the strength of powder metallurgical products is strongly requested. In particular, there is a strong request for strengthening iron-based powder products (iron-based sintered bodies).

Generally, an iron-based powder green compact for powder metallurgy which is a former stage of an iron-based sintered body is produced by adding to an iron-based powder, an alloying powder such as copper powder and graphite powder, and a lubricant such as stearic acid and zinc stearate to obtain an iron-based mixed powder, injecting said powder into a die and performing pressing. Based on the components, iron-based powders are categorized into iron powder (e.g. pure iron powder and the like), alloy steel powder, and the like. Further, when categorized by production method, iron-based powders are categorized into atomized iron powder, reduced iron powder, and the like. Within these categories, the term “iron powder” is used with a broad meaning encompassing alloy steel powder.

The density of an iron-based powder green compact for powder metallurgy which is obtained in a general powder metallurgy process is normally around 6.8 Mg/m³ to 7.3 Mg/m³. The obtained iron-based powder green compact is then sintered to form an iron-based sintered body which in turn is further subjected to optional sizing, cutting work or the like to form a powder metallurgical product. Further, when an even higher strength is required, carburizing heat treatment or bright heat treatment may be performed after sintering.

Conventionally known powders with an alloying element added thereto at the stage of precursor powder include (1) mixed powder obtained by adding each alloying element powder to pure iron powder, (2) pre-alloyed steel powder obtained by completely alloying each element, (3) diffusionally adhered alloy steel powder obtained by partially diffusing each alloying element powder on the surface of pure iron powder or pre-alloyed steel powder, and the like.

The mixed powder (1) obtained by adding each alloying element powder to pure iron powder is advantageous in that high compressibility equivalent to that of pure iron powder can be obtained. However, the large segregation of each alloying element powder would cause a large variation in characteristics. Further, since the alloying elements do not sufficiently diffuse in Fe, the microstructure would remain non-uniform and the matrix would not be strengthened efficiently.

Therefore, the mixed powder obtained by adding each alloying element powder to pure iron powder could not cope with the recent requests for stabilizing characteristics and increasing strength, and the usage amount thereof is decreasing.

Further, the pre-alloyed steel powder (2) obtained by completely alloying each element is produced by atomizing molten steel, and although the matrix is strengthened by a uniform microstructure, a decrease in compressibility is caused by the action of solid solution hardening.

Further, the diffusionally adhered alloy steel powder (3) is produced by adding metal powders of each element to pure iron powder or pre-alloyed steel powder, heating the resultant powder in a non-oxidizing or reducing atmosphere, and partially diffusion bonding each metal powder on the surfaces of the pure iron powder or the pre-alloyed steel powder, and advantages of the iron-based mixed powder (1) and the pre-alloyed steel powder (2) can be combined.

Therefore, high compressibility equivalent to that of pure iron powder can be obtained while preventing segregation of alloying elements. Further, since a multi-phase where partially concentrated alloy phase is diffused is formed, the matrix may be strengthened. For these reasons, development is carried out for diffusionally adhered alloy steel powder for high strength.

As described above, increasing the alloy content is one way of enhancing strength and toughness of a powder metallurgical product. However, such alloying hardens the alloy steel powder to be used as the material, leading to the problem of decreased compressibility and increased burden on the equipment for performing pressing. Further, the decrease in compressibility of the alloy steel powder causes a decrease in density of the sintered body, which ends up canceling the increase in strength. Therefore, in order to increase the strength and toughness of powder metallurgical products, a technique is required for increasing the strength of the sintered body while minimizing the decrease in compressibility.

As a technique for increasing the strength of the sintered body while maintaining compressibility such as mentioned above, a technique of adding to the iron-based powder, alloying elements such as Ni, Cu, Mo and the like which improve hardenability, is commonly used. As an element that is effective for this purpose, for example, PTL1 (JPS6366362B) discloses a technique of adding Mo as a pre-alloyed element to the iron powder in a range that would not deteriorate compressibility (Mo: 0.1 mass % to 1.0 mass %), and diffusionally adhering, to the particle surfaces of the resultant iron powder, powders of Cu and Ni to achieve both compressibility during green compacting and strength of members after sintering.

Further, PTL2 (JPS61130401A) proposes an alloy steel powder for powder metallurgy for a high strength sintered body obtained by diffusionally adhering, to the steel powder surface, two or more kinds of alloying elements, in particular Mo and Ni, or Cu in addition to said elements.

With this technique, it is further proposed that, for each diffusionally adhered element, the diffusionally adhered density with respect to fine powders of particle sizes of 44 μm or less is controlled within a range of 0.9 to 1.9 times the diffusionally adhered density with respect to the total amount of the steel powder, and it is disclosed that with a limitation to such relatively broad range, impact toughness of the sintered body is obtained.

On the other hand, Mo based alloy steel powder containing Mo as a main alloying element and not containing Ni or Cu has been proposed. For example, in PTL3 (JPH0689365B), an alloy steel powder containing Mo which is a ferrite-stabilizing element as a pre-alloy in a range of 1.5 mass % to 20 mass % is proposed to accelerate sintering by forming an a single phase of Fe having a rapid self diffusion rate. It is disclosed that, with this alloy steel powder, a sintered body with high density is obtained by applying particle size distribution and the like in the process referred to as pressure sintering, and a uniform and stable microstructure is obtained by not employing a diffusionally adhered alloying element.

Similarly, PTL4 (JP2002146403A) discloses a technique regarding an alloy steel powder for powder metallurgy containing Mo as a main alloying element. This technique proposes an alloy steel powder obtained by diffusionally adhering 0.2 mass % to 10.0 mass % of Mo on the surface of the iron-based powder containing, as a pre-alloy, 1.0 mass % or less of Mn, or less than 0.2 mass % of Mo. It is disclosed that, atomized iron powder or reduced iron powder may be used as the iron-based powder, and that the mean particle size is preferably 30 μm to 120 p.m. Further, it is disclosed that the alloy steel powder not only has excellent compressibility but also enables obtaining sintered parts having high density and high strength.

CITATION LIST Patent Literature

PTL 1: JPS6366362B

PTL 2: JPS61130401A

PTL 3: JPH0689365B

PTL 4: JP2002146403A

SUMMARY Technical Problem

However, while Ni is an essential additive component for the techniques disclosed in PTL1 and PTL2, the diffusion of Ni progresses at a slow rate during sintering, and therefore sintering needs to be performed for a long period of time for sufficiently diffusing Ni in iron powder or steel powder.

Further, with the technique disclosed in PTL3, since Mo is added in a relatively large amount of 1.8 mass % or more and the compressibility is low, high forming density cannot be obtained. Therefore, when a normal sintering process (single sintering with no pressurization) is applied, only sintered parts having low sintered density can be obtained, and sufficient strength and toughness cannot be obtained.

Further, the technique disclosed in PTL4 is applied to a powder metallurgy process comprising re-compression and re-sintering of the sintered body. In other words, with a normal sintering method, the aforementioned effect could not sufficiently be achieved.

As a result of our study, it was revealed that it is difficult to achieve both high strength and high toughness with a sintered body using any alloy steel powder disclosed in the above PTLs 1 to 4.

It could therefore be helpful to provide an alloy steel powder for powder metallurgy that can solve the aforementioned problem and achieve both high strength and high toughness of the sintered body using the alloy steel powder.

Solution to Problem

To achieve the above object, we made intensive studies regarding the alloy components of the iron-based powder and the adding means thereof, and discovered the following.

That is, we discovered that, by using a composite alloy steel powder obtained by adhering an Mo-containing alloy powder to a surface of an iron-based powder, wherein the composite alloy steel powder has a specific surface area of 0.100 m²/g or more and Mo content of 0.2 mass % to 1.5 mass %, pores of the sintered body are appropriately refined because of the excellent sinterability that the alloy steel powder for powder metallurgy obtained from the composite alloy steel powder offers during pressing and sintering, and accordingly the strength of the sintered body as well as the toughness of the sintered body are improved.

This disclosure has been made based on these discoveries.

We thus provide:

1. An alloy steel powder for powder metallurgy comprising: a composite alloy steel powder obtained by adhering an Mo-containing alloy powder to a surface of an iron-based powder; and graphite powder, wherein the composite alloy steel powder has a specific surface area of 0.100 m²/g or more and Mo content in a range of 0.2 mass % to 1.5 mass %, and the graphite powder content with respect to 100 mass % of the alloy steel powder for powder metallurgy is in a range of 0.1 mass % to 1.0 mass %.

2. The alloy steel powder for powder metallurgy according to aspect 1, further containing Cu powder in a range of 0.5 mass % to 4.0 mass % with respect to 100 mass % of the alloy steel powder for powder metallurgy.

3. The alloy steel powder for powder metallurgy according to aspect 1 or 2, wherein the iron-based powder contains a reduced iron powder, and a mean particle size of the iron-based powder is 80 μm or less.

4. The alloy steel powder for powder metallurgy according to any one of aspects 1 to 3, wherein oxygen content of the iron-based powder is 0.3 mass % or less.

5. A method of producing an iron-based sintered body comprising: adding and mixing a lubricant into the alloy steel powder for powder metallurgy according to any one of aspects 1 to 4; and performing pressing and sintering to obtain an iron-based sintered body.

Advantageous Effect

With the alloy steel powder for powder metallurgy described herein, since Ni is not required and compressibility is high, a sintered material having both high strength and high toughness can be obtained at a low cost, even with a normal sintering method.

DETAILED DESCRIPTION

Our methods and products will be described in detail below. The alloy steel powder for powder metallurgy described herein is obtained using a composite alloy steel powder that is obtained by adhering an Mo-containing alloy powder to the surface of an iron-based powder, wherein the composite alloy steel powder has a specific surface area of 0,100 m²/g or more and Mo content in a range of 0.2 mass % to 1.5 mass %.

Further, by mixing the above composite alloy steel powder with an appropriate amount described below of graphite powder, the alloy steel powder for powder metallurgy is obtained. Then, by pressing the alloy steel powder for powder metallurgy into a green compact, and sintering said green compact, pores of the sintered body are effectively refined and sintered parts with improved strength and toughness can be obtained.

The mechanism by which pores of the sintered body can be effectively refined and sintered parts with improved strength and toughness can be obtained is understood as follows.

Generally, many pores exist in a sintered body, and therefore stress concentrates in pore parts and tends to cause a decrease in strength or toughness of the sintered body. However, with the alloy steel powder for powder metallurgy described herein, by setting the specific surface area of the composite alloy steel powder to 0.100 m²/g or more, the pores in the sintered body are refined, the degree of stress concentration is mitigated and the sintered neck part is toughened.

In addition, by setting the Mo content of the composite alloy steel powder in a range of 0.2 mass % to 1.5 mass %, Mo concentrates in the pore surrounding part of the sintered body, and the sintered body is further strengthened. Further, since Mo-containing alloy powder is adhered to the surface of the iron-based powder for the alloy steel powder for powder metallurgy described herein and Mo is not contained in the matrix part, carbide is less likely generated compared to the sintered neck part, and therefore a microstructure with high toughness is obtained.

In other words, it is believed that by controlling pore distribution and Mo distribution of the sintered body, both high strength and high toughness of the sintered body were made achievable.

The reasons for the limitations of the disclosure are described below. First, the method of producing an alloy steel powder for powder metallurgy described herein will be explained.

In the disclosure, iron powders such as atomized iron powder and reduced iron powder as the iron-based powder, and Mo material powder which is the material of Mo-containing alloy powder are prepared.

The above iron-based powder is not particularly limited as long as it is an iron-based powder which is normally used for powder metallurgical techniques. However, the so-called as-atomized powder, atomized iron powder, or reduced iron powder is preferable. As the atomized iron-based powder, either one of as-atomized powder obtained by atomizing molten steel and then drying and classifying the resulting powder, or atomized iron powder obtained by reducing as-atomized powder in a reductive atmosphere, may be used.

Further, as reduced iron powder, it is preferable to use reduced iron powder obtained by reducing mill scale generated during production of steel materials or iron ore. The apparent density of the reduced iron powder may be around 1.7 Mg/m³ to 3.0 Mg/m³. More preferably, it is 2.2 Mg/m³ to 2.8 Mg/m³. Here, the apparent density is measured by the test method of JIS Z 2504.

Meanwhile, as the Mo material powder, the desired Mo-containing alloy powder itself may be used, or an Mo compound that can be reduced to Mo-containing alloy powder can be used. The mean particle size of the Mo material powder is 50 μm or less, and preferably 20 μm or less. Here, the mean particle size refers to the volume-based median size (so-called d50).

As the Mo-containing alloy powder, Mo alloy powders including pure metal powder of Mo, oxidized Mo powder, Fe—Mo (ferromolybdenum) powder and the like are advantageously applied. On the other hand, examples of an Mo compound include Mo carbide, Mo sulfide, Mo nitride and the like.

Then, the above iron-based powder and Mo material powder are mixed in a predetermined ratio to obtain a mixed powder. This ratio is adjusted so that the final Mo content of the composite alloy steel powder is in a range of 0.2 mass % to 1.5 mass %. For the mixing, the mixing method or mixing facility is not particularly limited, and the powders may be mixed in accordance with conventional methods using a Henschel mixer, a cone mixer or the like.

Further, by maintaining the mixed powder at a high temperature, diffusing and bonding Mo to steel in the contact surface of the iron-based powder and the Mo material powder (diffusing-bonding treatment), the composite alloy steel powder used herein is obtained.

As the atmosphere for the diffusion-bonding treatment, reductive atmosphere or hydrogen containing atmosphere is preferable, and hydrogen containing atmosphere is particularly suitable. The heat treatment may be performed under vacuum. Further, a preferred temperature for diffusion-bonding treatment is in a range of 800° C. to 1000° C.

When diffusion-bonding treatment is performed as mentioned above, the iron-based powder and the Mo-containing alloy powder are normally in the state where they are sintered and agglomerated. Therefore, they are ground and classified into desired particle sizes. Further, annealing may optionally be performed. The particle size of the composite alloy steel powder is preferably 180 μm or less.

In the disclosure, the Mo-containing alloy powder is preferably adhered uniformly to the surface of the iron-based powder. If not adhered uniformly, Mo-containing alloy powder tends to come off from the surface of the iron-based powder in situations such as when grinding the composite alloy steel powder after diffusion-bonding treatment, or during transportation thereof, and therefore Mo-containing alloy powder in a free state increases particularly easily. When pressing the alloy steel powder for powder metallurgy obtained from the composite alloy steel powder in such state to obtain a green compact and sintering the green compact, there may be segregation of the dispersed carbide. Therefore, to enhance the strength and toughness of the sintered body, it is preferable to uniformly adhere the Mo-containing alloy powder on the surface of the iron-based powder to reduce the Mo-containing alloy powder in a free state resulting from coming off or the like.

The content of Mo to be diffusionally adhered is in the range of 0.2 mass % to 1.5 mass % (included number) with respect to the total amount of the composite alloy steel powder. This is because while if said content falls under 0.2 mass %, both the hardenability improving effect and the strength improving effect are reduced, if said content exceeds 1.5%, the hardenability improving effect reaches a plateau, and causes an increase in the non-uniformity of the microstructure of the sintered body, and high strength and toughness cannot be obtained. Therefore, the content of Mo to be diffusionally adhered is in a range of 0.2 mass % to 1.5 mass % with respect to the total amount of the composite alloy steel powder. Preferred content is in the range of 0.3 mass % to 1.0 mass %.

On the other hand, the specific surface area of the composite alloy steel powder which is obtained by diffusing and adhering Mo thereon is limited to 0.100 m²/g or more. The area is preferably 0.150 m²/g or more. This is because a specific surface area of less than 0.100 m²/g provides coarse pores or insufficient reactivity during sintering, or for both reasons, which fact will result in little progress in refinement of pores and decreased toughness. Although the upper limit of the specific surface area is not particularly limited, if said area exceeds 0.5 m²/g, a large amount of fine powder will be included and compressibility will decrease. Therefore, the specific surface area is preferably 0.5 m²/g or less.

Further, since the specific surface area of the alloy steel powder will be reduced by diffusionally adhering Mo to the surface of the iron-based powder, the specific surface area of the iron-based powder as the base is preferably 0.150 m²/g or more. The specific surface area used herein is measured by a gas adsorption method (BET method).

In the disclosure, the balance of the composite alloy steel powder is iron and incidental impurities. Examples of impurities contained in the composite alloy steel powder include C, O, N, and S. As long as the contents of these components in the composite alloy steel powder are limited to C: 0.02 mass % or less, O: 0.3 mass % or less, N: 0.004 mass % or less, and S: 0.03 mass % or less, there is no particular problem. 0 content is preferably 0.25% or less. This is because if the amount of incidental impurities exceeds the above ranges, the compressibility of the alloy steel powder for powder metallurgy obtained from the composite alloy steel powder decreases, and it becomes difficult to perform compression molding to form a preformed body having a sufficient density.

For the alloy steel powder for powder metallurgy containing the above composite alloy steel powder as the main component, it is important that graphite powder is added in a range of 0.1 mass % to 1.0 mass % in a ratio with respect to the total amount of the alloy steel powder for powder metallurgy (100 mass %). Further, in the disclosure, 0.5 mass % to 4.0 mass % of Cu powder can be added in a ratio with respect to the total amount of the alloy steel powder for powder metallurgy (100 mass %).

C, which is a main component of graphite powder, dissolves in iron during sintering and enables achieving solid solution strengthening and hardenability improvement, and therefore C is a useful element for enhancing the strength of sintered parts. In a case where carburizing heat treatment or the like is performed after sintering and the sintered body is carburized from the outside, the amount of graphite powder added may be small. However, if it is less than 0.1 mass %, the above effect obtained by adding graphite powder is limited. On the other hand, graphite powder will also be added when carburizing heat treatment is not performed during sintering. However, if the amount of graphite powder added exceeds 1.0 mass %, the sintered body becomes hypereutectoid, and cementite is precipitated and causes a decrease in strength. Therefore, the amount of graphite powder is limited to a range of 0.1 mass % to 1.0 mass %. The mean particle size of graphite powder is preferably 50 μm or less.

On the other hand, Cu is a useful element that achieves solid solution strengthening of the iron-based powder and has an improving effect in hardenability of the iron-based powder, thereby enhancing the strength of sintered parts. Cu melts into a liquid phase during sintering of iron-based powder, and has an effect of fixing iron-based powder particles to one another. However, if the amount of Cu powder added is less than 0.5 mass %, the addition effect is limited. On the other hand, if it exceeds 4.0 mass %, not only does the strength improving effect of the sintered parts reach a plateau but also leads to a decrease in cuttability. Therefore, the amount of Cu powder is preferably in a range of 0.5 mass % to 4.0 mass %, and more preferably in a range of 1.0 mass % to 3.0 mass %. The mean particle size of Cu powder is preferably 50 μm or less.

The iron-based powder used herein contains reduced iron powder and the mean particle size of the iron-based powder is preferably 80 μm or less. This is because if powder with a mean particle size of larger than 80 μm i.e. powder with a large particle size is included, the driving force during sintering weakens and coarse holes are formed around the coarse iron-based powder. These coarse holes become the cause of reducing the strength and toughness of the sintered body.

Here, the above mean particle size refers to the mass-based median size (so-called d50). In detail, the iron-based powder is sieved using a sieve defined by JIS Z 8801, the mass of the sample powder remaining on each sieve was measured, and a particle size where the amounts of small particles and large particles become equal was obtained and defined as the mean particle size.

In this disclosure, additives for improving characteristics may be added in accordance with the purpose. For example, Ni powder may be added as necessary for the purpose of improving the strength of the sintered body, and powders for improving machinability such as MnS may be added as necessary for the purpose of improving cuttability of the sintered body. Ni powder is preferably in a range of 0.5 mass % to 5 mass % in a ratio with respect to the total amount of the alloy steel powder for powder metallurgy (100 mass %).

On the other hand, the additive amount of powders for improving machinability such as MnS may be a conventionally known additive amount i.e. around 0.1 mass % to 1 mass % in a ratio with respect to the total amount of the alloy steel powder for powder metallurgy (100 mass %).

Further, preferable pressing conditions and sintering conditions for producing a sintered body using the alloy steel powder for powder metallurgy described herein will be explained.

When performing pressing using the alloy steel powder for powder metallurgy described herein, a lubricant powder may also be mixed in. Further, pressing may be performed by applying or adhering a lubricant to a die. In either case, as the lubricant, metal soap such as zinc stearate and lithium stearate, amide-based wax such as ethylenebisstearamide, and other well known lubricants may all be used suitably. When mixing the lubricant, the amount thereof is preferably around 0.1 parts by mass to 1.2 parts by mass (externally added) with respect to 100 parts by mass of the alloy steel powder for powder metallurgy.

Pressing of the alloy steel powder for powder metallurgy described herein is preferably performed with a pressure of 400 MPa to 1000 MPa. This is because if the pressure is less than 400 MPa, the density of the obtained green compact lowers and leads to a decrease in characteristics of the sintered body, whereas if it exceeds 1000 MPa, life of the die shortens and becomes economically disadvantageous. The pressing temperature is preferably in the range of room temperature (around 20° C.) to around 160° C.

Further, the alloy steel powder for powder metallurgy described herein is sintered preferably in a temperature range of 1100° C. to 1300° C. This is because if the sintering temperature is lower than 1100° C., progressing of sintering stops and leads to a decrease in characteristics of the sintered body, whereas if it exceeds 1300° C., life of the sintering furnace shortens and becomes economically disadvantageous. The sintering time is preferably in the range of 10 minutes to 180 minutes.

The obtained sintered body can optionally be subjected to strengthening treatment such as carburizing-quenching, bright quenching, induction hardening, and carburizing nitriding treatment. However, even if strengthening treatment is not performed, the sintered body obtained using the alloy steel powder for powder metallurgy described herein has improved strength and toughness compared to conventional sintered bodies (which are not subjected to strengthening treatment). Each strengthening treatment may be performed in accordance with conventional methods.

Examples

Although the disclosure will be described below in further detail with reference to examples, the disclosure is not intended to be limited in any way to the following examples.

As iron-based powders, an as-atomized powder with an apparent density of 2.65 Mg/cm³, 2.80 Mg/m³, or 3.25 Mg/m³, a reduced iron powder with an apparent density of 2.60 Mg/cm³ or 2.75 Mg/m³, and an atomized iron powder with an apparent density of 2.60 Mg/m³, 2.80 Mg/m³, or 3.30 Mg/m³ were used.

Oxidized Mo powder (mean particle size: 10 μm) was added to these iron-based powders at a predetermined ratio, and the resultant powders were mixed for 15 minutes in a V-shaped mixer to obtain a mixed powder, then the mixed powder was subjected to heat treatment in a hydrogen atmosphere with a drew point of 30° C. (holding temperature: 880° C., holding time: 1 h) to diffuse and adhere Mo to surfaces of the iron-based powders to obtain a composite alloy steel powder. Mo content with respect to the composite alloy steel powder is shown in table 1.

Then, copper powder (mean particle size: 30 μm) and graphite powder (mean particle size: 5 μm) in the amounts shown in table 1 were added to the composite alloy steel powder (Mo-diffusionally adhered alloy steel powder) to obtain an alloy steel powder for powder metallurgy. Then, 0.6 parts by mass of ethylenebisstearamide was added with respect to 100 parts by mass of the alloy steel powder for powder metallurgy, and then the resulting powders were mixed in a V-shaped mixer for 15 minutes. Subsequently, the powders were pressed until the density of the resulting green compacts reached 7.0 Mg/m³, and tablet shaped green compacts with length of 55 mm, width of 10 mm, and thickness of 10 mm were produced.

The tablet shaped green compacts were sintered to obtain sintered bodies. Sintering was performed in propane converted gas atmosphere at a sintering temperature of 1130° C., for a sintering time of 20 minutes.

Then, the obtained sintered bodies were processed into round bar tensile test specimens, each having a parallel portion diameter of 5 mm, for a tensile test specified in JIS Z 2241. For Charpy impact tests specified in JIS Z 2242, the obtained sintered bodies with shapes as sintered which were subjected to gas carburizing of carbon potential of 0.8 mass % (holding temperature: 870° C., holding time: 60 minutes), then quenching (60° C., oil quenching) and tempering (180° C., 60 minutes) were used.

The sintered bodies were subjected to tensile tests specified in JIS Z 2241, and Charpy impact tests defined by JIS Z 2242 to measure the tensile strength (MPa) and the impact value (J/cm²). The measurement results of each sintered body are shown in Table 1.

TABLE 1 Apparent Mean Particle Specific Surface Tensile Impact Density *1 Size *2 Oxygen Content *3 Mo Area *4 Cu Graphite Strength Value Material Mg/m³ μm mass % mass % m²/g mass % mass % MPa J/cm² Remarks As-Atomized 2.65 76 0.58 1.4 0.152 1.0 0.5 1220 15.4 Example 1 Powder As-Atomized 2.65 80 0.60 1.0 0.145 0.5 0.3 1124 14.1 Example 2 Powder As-Atomized 2.80 85 0.56 0.8 0.120 2.0 0.3 1150 15.2 Example 3 Powder As-Atomized 2.80 84 0.55 0.6 0.118 3.0 0.5 1120 15.5 Example 4 Powder Reduced Iron 2.60 79 0.25 0.4 0.124 4.0 0.7 1163 15.3 Example 5 Powder Reduced Iron 2.60 78 0.27 0.2 0.121 2.0 0.5 1105 15.7 Example 6 Powder Reduced Iron 2.75 96 0.28 1.4 0.105 1.5 0.5 1174 15.9 Example 7 Powder Reduced Iron 2.75 94 0.31 0.6 0.103 3.0 1.0 1200 14.9 Example 8 Powder As-Atomized 2.80 94 0.57 0.7 0.109 2.0 0.1 1006 14.0 Example 9 Powder Atomized Iron 2.60 77 0.12 1.2 0.103 — 0.5 1010 14.1 Example 10 Powder As-Atomized 2.80 95 0.62 0.1 0.115 1.0 0.3 880 12.7 Comparative Powder Example 1 As-Atomized 3.25 96 0.08 1.0 0.080 2.0 0.3 1103 12.9 Comparative Powder Example 2 Reduced Iron 2.75 106 0.35 0.6 0.090 1.5 1.1 940 12.8 Comparative Powder Example 3 Atomized Iron 2.80 102 0.12 1.7 0.103 3.5 0.7 1040 12.4 Comparative Powder Example 4 Atomized Iron 2.80 95 0.13 0.8 0.105 4.5 1.0 1212 14.0 Example 11 Powder Atomized Iron 3.30 104 0.15 0.4 0.098 0.5 0.5 1047 9.8 Comparative Powder Example 5 As-Atomized 2.80 96 0.56 *5 — — 0.3 998 13.3 Conventional Powder Example *1 Apparent density of iron-based powder *2 Mean particle size of iron-based powder *3 Oxygen content of iron-based powder *4 Specific surface area of Mo-diffusionally adhered alloy steel powder *5 4% Ni—1.5% Cu—0.5% Mo

As shown in Table 1, when comparing the tensile strength and impact value of our examples with comparative examples, our examples all showed tensile strength of 1000 MPa or more and impact value of 14.0 J/cm² or more, and both high strength and high toughness were achieved, whereas the comparative examples all showed impact values of less than 14.0 J/cm² and were poor in at least one of tensile strength and impact value compared to our examples.

Table 1 also shows the results of a 4Ni material (4Ni-1.5Cu-0.5Mo, maximum particle size of material powder: 180 μm) as the conventional material. It can be seen that our examples exhibit better characteristics over the conventional 4Ni material. 

1. An alloy steel powder for powder metallurgy comprising: a composite alloy steel powder obtained by adhering an Mo-containing alloy powder to a surface of an iron-based powder; and graphite powder, wherein the composite alloy steel powder has a specific surface area of 0.100 m²/g or more and Mo content in a range of 0.2 mass % to 1.5 mass %, and the graphite powder content with respect to 100 mass % of the alloy steel powder for powder metallurgy is in a range of 0.1 mass % to 1.0 mass %.
 2. The alloy steel powder for powder metallurgy according to claim 1, further containing Cu powder in a range of 0.5 mass % to 4.0 mass % with respect to 100 mass % of the alloy steel powder for powder metallurgy.
 3. The alloy steel powder for powder metallurgy according to claim 1, wherein the iron-based powder contains a reduced iron powder, and a mean particle size of the iron-based powder is 80 μm or less.
 4. The alloy steel powder for powder metallurgy according to claim 1, wherein oxygen content of the iron-based powder is 0.3 mass % or less.
 5. A method of producing an iron-based sintered body comprising: adding and mixing a lubricant into the alloy steel powder for powder metallurgy according to claim 1; and performing pressing and sintering to obtain an iron-based sintered body.
 6. The alloy steel powder for powder metallurgy according to claim 2, wherein the iron-based powder contains a reduced iron powder, and a mean particle size of the iron-based powder is 80 μm or less.
 7. The alloy steel powder for powder metallurgy according to claim 2, wherein oxygen content of the iron-based powder is 0.3 mass % or less.
 8. The alloy steel powder for powder metallurgy according to claim 3, wherein oxygen content of the iron-based powder is 0.3 mass % or less.
 9. The alloy steel powder for powder metallurgy according to claim 6, wherein oxygen content of the iron-based powder is 0.3 mass % or less.
 10. A method of producing an iron-based sintered body comprising: adding and mixing a lubricant into the alloy steel powder for powder metallurgy according to claim 2; and performing pressing and sintering to obtain an iron-based sintered body.
 11. A method of producing an iron-based sintered body comprising: adding and mixing a lubricant into the alloy steel powder for powder metallurgy according to claim 3; and performing pressing and sintering to obtain an iron-based sintered body.
 12. A method of producing an iron-based sintered body comprising: adding and mixing a lubricant into the alloy steel powder for powder metallurgy according to claim 4; and performing pressing and sintering to obtain an iron-based sintered body.
 13. A method of producing an iron-based sintered body comprising: adding and mixing a lubricant into the alloy steel powder for powder metallurgy according to claim 6; and performing pressing and sintering to obtain an iron-based sintered body.
 14. A method of producing an iron-based sintered body comprising: adding and mixing a lubricant into the alloy steel powder for powder metallurgy according to claim 7; and performing pressing and sintering to obtain an iron-based sintered body.
 15. A method of producing an iron-based sintered body comprising: adding and mixing a lubricant into the alloy steel powder for powder metallurgy according to claim 8; and performing pressing and sintering to obtain an iron-based sintered body.
 16. A method of producing an iron-based sintered body comprising: adding and mixing a lubricant into the alloy steel powder for powder metallurgy according to claim 9; and performing pressing and sintering to obtain an iron-based sintered body. 